Electric motors are subject to the risk of the motor winding overheating, overloads, poor network qualities (low voltage/excess voltage), impaired cooling, a blocked rotor, and the like, being among the most frequent causes of malfunction. Motors, in particular refrigeration compressor motors which are heavily used and are therefore subjected to a forced cooling operation, must be protected from such risks.
DE 198 22 056 A1 discloses a temperature protection circuit for protecting electrical devices having a series connection of PTC thermistors having stepped nominal response temperatures. The characteristic line of PTC thermistors is relatively flat from ambient temperature to a few degrees Celsius below the nominal response temperature NAT and then increases exponentially. A statement relating to the winding temperature is not possible with such PTC thermistors. However, the change of the resistance value in the range of the nominal response temperature NAT can be detected by an evaluation device of an actuation device. The switching-off operation of the motor is initiated when the evaluation device has determined that the nominal response temperature NAT has been exceeded.
Although PTC sensors are very small, as a result of the thermal transition resistances between the sensor and the winding, they can follow the very rapid temperature increase only in a delayed manner. In the case of refrigeration compressor motors with winding current densities up to 80 A/mm2, in the event of a short-circuit the temperatures increase at 25-30 K/s. This means that delays of a few seconds in switching off can already produce a very high temperature overshoot. The switching-off is carried out only when the temperature detected has reached the nominal response temperature. At this time, however, the actual winding temperature is already substantially over the predetermined threshold value.
EP 2 187 494 B1 therefore proposes a protection device for electric motors having a sensor circuit having at least a first PTC sensor and at least a second PTC sensor, the nominal response temperatures having different magnitudes. The two PTC sensors are connected to each other in such a manner that the total resistance of the sensor circuit has a characteristic line which in each case provides for an inflection point in the range of the nominal response temperatures. With such a characteristic line, the reaching of a critical situation can be detected even more rapidly.
Although PTC thermistors have been found to be effective as protection against excess temperature, there are applications in which they are disadvantageous. This is particularly the case when an unusually rapid temperature increase occurs. In this instance, a conventional actuation when the nominal response temperature is exceeded would lead to a large temperature overshoot which could have dangerous consequences.
Furthermore, each safety shutdown operation represents a loss of the availability of the installation and can consequently have significant consequences. In the case of highly loaded motors in which the availability of the installation which is operated thereby is very important, it is desirable, when an alarming temperature range is reached, to introduce further measures in order to prevent a further temperature increase in order to prevent a safety shutdown operation when the nominal response temperature is reached. To this end, corresponding proposals are set out in DE 10 2005 052 042. However, the implementation of this additional function generally requires the use of additional measurement channels or devices.
DE 602 05 697 T2 relates to a semi-conductor device having a temperature change detector and a temperature corrector, sense resistors of a type A and sense resistors of a type B being used, the sense resistors of the type A having a positive temperature coefficient and a linearly increasing resistance value and the sense resistors of the type B being characterised by a non-linear resistance line which gradually decreases with the temperature. A precise temperature measurement is neither provided nor conceivable with this temperature change detector.